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Abstract:

There are provided a plasma doping method and an apparatus which have
excellent reproducibility of the concentration of impurities implanted
into the surfaces of samples. In a vacuum container, in a state where gas
is ejected toward a substrate placed on a sample electrode through gas
ejection holes provided in a counter electrode, gas is exhausted from the
vacuum container through a turbo molecular pump as an exhaust device, and
the inside of the vacuum container is maintained at a predetermined
pressure through a pressure adjustment valve, the distance between the
counter electrode and the sample electrode is set to be sufficiently
small with respect to the area of the counter electrode to prevent plasma
from being diffused outward, and capacitive-coupled plasma is generated
between the counter electrode and the sample electrode to perform plasma
doping. The gas used herein is a gas with a low concentration which
contains impurities such as diborane or phosphine.

Claims:

1. A plasma doping method comprising: placing a substrate on a first
electrode within a vacuum chamber; supplying an electric power to the
first electrode, while supplying a plasma doping gas into the vacuum
chamber, exhausting gas from the vacuum chamber, and controlling an
inside of the vacuum chamber to a predetermined pressure, and generating
plasma between a surface of the substrate and a surface of a second
electrode within the vacuum chamber; supplying a high-frequency electric
power to the second electrode which is placed opposite the first
electrode; and performing plasma doping processing to implant impurities
into the surface of the substrate, in a state where a following equation
(1) is satisfied, where S: an area of the surface which is faced to the
second electrode, out of surfaces of the substrate, and G: a distance
between the first electrode and the second electrode. 0.1 {square root
over ((S/π))}G0.4 {square root over ((S/π))} (1)

2. The plasma doping method as claimed in claim 1, further comprising,
after the substrate is placed on the first electrode within the vacuum
chamber and before the electric power is supplied to the first electrode,
supplying a high-frequency electric power is supplied to the second
electrode while a pressure within the vacuum chamber is maintained at a
plasma generating pressure which is higher than the predetermined
pressure, to generate plasma between the surface of the substrate and the
surface of the second electrode within the vacuum chamber, gradually
decreasing a pressure within the vacuum chamber to the predetermined
pressure after the plasma is generated, and supplying the electric power
to the first electrode after the pressure within the vacuum chamber
reaches the predetermined pressure.

3. The plasma doping method as claimed in claim 1, further comprising,
after the substrate is placed on the first electrode within the vacuum
chamber and before the electric power is supplied to the first electrode,
supplying a plasma generating gas which causes discharge at a lower
pressure more easily than a dilution gas used for diluting an impurity
material gas in the plasma doping gas into the vacuum chamber, supplying
the high-frequency electric power to the second electrode while the
pressure within the vacuum chamber is maintained at the predetermined
pressure, generating plasma between the surface of the substrate and the
surface of the second electrode within the vacuum chamber, switching a
gas supplied into the vacuum chamber to the plasma doping gas after the
plasma is generated, and supplying the electric power to the first
electrode after the gas inside the vacuum chamber has been switched to
the plasma doping gas.

4. The plasma doping method as claimed in claim 1, wherein, after the
substrate is placed on the first electrode within the vacuum chamber and
before the electric power is supplied to the first electrode, relatively
moving the first electrode and the second electrode to separate the first
electrode from the second electrode such that the distance G between the
first electrode and the second electrode is larger than a range defined
by the equation (1), and in this state, supplying the high-frequency
electric power to the second electrode while a plasma doping gas is
supplied into the vacuum chamber, gas is exhausted from the vacuum
chamber, and the inside of the vacuum chamber is controlled to the
predetermined pressure, generating plasma between the surface of the
substrate and the surface of the second electrode within the vacuum
chamber, relatively moving the first electrode and the second electrode
after the plasma is generated to restore a state where the distance G
satisfies the equation (1), and thereafter, supplying the electric power
to the first electrode.

5. The plasma doping method as claimed in claim 1, wherein a
concentration of impurity material gas within the gas introduced into the
vacuum chamber is equal to or less than 1%.

6. The plasma doping method as claimed in claim 1, wherein a
concentration of impurity material gas within the gas introduced into the
vacuum chamber is equal to or less than 0.1%.

7. The plasma doping method as claimed in claim 1, wherein the gas
introduced into the vacuum chamber is a mixed gas prepared by diluting an
impurity material gas with a rare gas.

8. The plasma doping method as claimed in claim 7, wherein the rare gas
is He.

9. The plasma doping method as claimed in claim 1, wherein the impurity
material gas within the gas is BxHy (x and y are natural numbers).

10. The plasma doping method as claimed in claim 1, wherein the impurity
material gas within the gas is PxHy (x and y are natural numbers).

11. The plasma doping method as claimed in claim 1, wherein the plasma
doping processing is performed while the gas is ejected toward the
surface of the substrate through gas ejection holes provided in the
second electrode.

12. The plasma doping method as claimed in claim 1, wherein the plasma
doping processing is performed in a state where the surface of the second
electrode is made of silicon or a silicon oxide.

13. The plasma doping method as claimed in claim 1, wherein the plasma
doping processing is performed in a state where the substrate is a
semiconductor substrate made of silicon.

14. The plasma doping method as claimed in claim 1, wherein impurities
within the impurity gas contained in the gas is arsenic, phosphorus, or
boron.

15-20. (canceled)

Description:

[0001] This is a continuation application of International Application No.
PCT/JP2007/069287, filed Oct. 2, 2007.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a plasma doping method and
apparatus for implanting impurities into the surfaces of samples.

[0003] For example, in fabrication of a MOS transistor, a thin oxide film
is formed on the surface of a silicon substrate as a sample, and then a
gate electrode is formed on the sample using a CVD apparatus or the like.
Thereafter, impurities are implanted thereto by a plasma doping method as
described above, using the gate electrode as a mask. By implanting
impurities, for example, a metal wiring layer is formed on the sample
where source and drain areas are formed in the sample to provide a MOS
transistor.

[0004] As a technique for implanting impurities into the surface of a
solid sample, there has been known a plasma doping method for implanting
ionized impurities into a solid with low energy (refer to Patent Document
1, for example). FIG. 5 illustrates the schematic structure of a plasma
processing apparatus for use in the plasma doping method as a
conventional impurity implantation method described in the aforementioned
Patent Document 1. In FIG. 5, there is provided a sample electrode 106
for placing thereon a sample 107 formed of a silicon substrate, in a
vacuum container 101. Within the vacuum container 101, there are provided
a gas supply device 102 for supplying a doping material gas containing
desired elements, such as B2H6, and a pump 108 for decreasing
the pressure within the vacuum container 101, which enables maintaining
the inside of the vacuum container 101 at a predetermined pressure. A
microwave waveguide 121 radiates a microwave into the vacuum container
101 through a quarts plate 122 as a dielectric window. Through the
interaction of the microwave and the DC magnetic field produced by an
electromagnet 123, there is formed a magnetic-field microwave plasma
(electron cyclotron resonance plasma) 124 within the vacuum container
101. A high-frequency power supply 112 is connected to the sample
electrode 106 through a capacitor 125, which enables controlling the
potential of the sample electrode 106. Further, the conventional distance
between the electrode and the quarts plate 122 is in the range of 200 to
300 mm.

[0005] In the plasma processing apparatus having such a structure, the
introduced doping material gas, such as B2H6, is changed into
plasma by the plasma generating means constituted by the microwave
waveguide 121 and the electromagnet 123, and boron ions in the plasma 124
are implanted into the surface of the sample 107 by the high-frequency
power supply 112.

[0006] As aspects of the plasma processing apparatus for use in plasma
doping, there are known one which uses a helicon-wave plasma source
(refer to Patent Document 2, for example), one which uses an
inductively-coupled plasma source (refer to Patent Document 3, for
example), and one which uses a parallel-plate plasma source (refer to
Patent Document 4, for example), as well as the aforementioned apparatus
which uses an electron cyclotron resonance plasma source. [0007] Patent
Document 1: U.S. Pat. No. 4,912,065 [0008] Patent Document 2: Japanese
Unexamined Patent Publication No. 2002-170782 [0009] Patent Document 3:
Japanese Unexamined Patent Publication No. 2004-47695 [0010] Patent
Document 4: Published Japanese translation of PCT International
Publication for Patent Application, No. 2002-522899

[0011] However, these conventional methods have an issue of poor
reproducibility of the amount of implanted impurities (the amount of
dose).

[0012] The present inventors have found, from various experiments, that
the poor reproducibility is caused by the increase in the density of
boron-based radicals within plasma. As plasma doping processing is
successively performed, a thin film containing boron (boron-based thin
film) is gradually deposited on the inner wall surface of the vacuum
container. It is considered that, in a case of using B2H6 as
the doping material gas, along with the increase in the thickness of the
deposited film, the probability of adsorption of boron-based radicals to
the inner wall surface of the vacuum container is gradually decreased,
and accordingly, the density of boron-based radicals in plasma is
gradually increased. Further, ions within plasma are accelerated by the
potential difference between the plasma and the inner wall of the vacuum
container and then impinge on the boron-based thin film deposited on the
inner wall surface of the vacuum container, thereby causing sputtering.
The sputtering thus caused gradually increases the amount of particles
containing boron which are supplied into the plasma. Consequently, the
amount of dose is gradually increased. The degree of the increase is
significantly large, and after plasma doping processing is repeatedly
carried out several hundreds of times, the amount of dose has been
increased to about 3.3 to 6.7 times the amount of dose that is implanted
in plasma doping processing performed just after the cleaning of the
inner wall of the vacuum container with water and an organic solvent.

[0013] Along with the generation of plasma and stoppage thereof, the
temperature of the inner wall surface of the vacuum container is varied,
which also changes the probability of adsorption of boron-based radicals
to the inner wall surface. This also causes the change in the amount of
dose.

[0014] The present invention is made in view of the aforementioned issues
in the prior art, and an object of the present invention is to provide a
plasma doping method and apparatus which are capable of controlling the
amount of impurities implanted to sample surfaces with higher accuracy
and providing highly reproducible impurity concentration.

SUMMARY OF THE INVENTION

[0015] In accomplishing these and other aspects, according to a first
aspect of the present invention, there is provided a plasma doping method
comprising:

[0016] placing a sample on a sample electrode within a vacuum container;

[0017] supplying an electric power to the sample electrode, while
supplying a plasma doping gas into the vacuum container, exhausting gas
from the vacuum container, and controlling an inside of the vacuum
container to a plasma doping pressure, and generating plasma between a
surface of the sample and a surface of a counter electrode within the
vacuum container; and

[0018] performing plasma doping processing to implant impurities into the
surface of the sample, in a state where a following equation (1) is
satisfied, where S is an area of the surface which is faced to the
counter electrode, out of surfaces of the sample, and G is a distance
between the sample electrode and the counter electrode.

0.1 {square root over ((S/π))}G0.4 {square root over ((S/π))} (1)

[0019] With this structure, it is possible to realize the plasma doping
method having excellent reproducibility of the concentration of
impurities implanted to the surfaces of samples.

[0020] According to a second aspect of the present invention, there is
provided the plasma doping method as defined in the first aspect, wherein
a high-frequency electric power is supplied to the counter electrode
which is placed opposite the sample electrode.

[0021] With this structure, it is possible to prevent the adsorption of
generated plasma to the counter electrode.

[0022] According to a third aspect of the present invention, there is
provided the plasma doping method as defined in the second aspect,
wherein, after the sample is placed on the sample electrode within the
vacuum container and before the electric power is supplied to the sample
electrode,

[0023] a high-frequency electric power is supplied to the counter
electrode while a pressure within the vacuum container is maintained at a
plasma generating pressure which is higher than the plasma doping
pressure, to generate plasma between the surface of the sample and the
surface of the counter electrode within the vacuum container, gradually
decreasing a pressure within the vacuum container to the plasma doping
pressure after the plasma is generated, and supplying the electric power
to the sample electrode after the pressure within the vacuum container
reaches the plasma doping pressure.

[0024] According to a fourth aspect of the present invention, there is
provided the plasma doping method as defined in the second aspect,
wherein, after the sample is placed on the sample electrode within the
vacuum container and before the electric power is supplied to the sample
electrode,

[0025] supplying a plasma generating gas which causes discharge at a lower
pressure more easily than a dilution gas used for diluting an impurity
material gas in the plasma doping gas into the vacuum container,
supplying the high-frequency electric power to the counter electrode
while the pressure within the vacuum container is maintained at the
plasma doping pressure, generating plasma between the surface of the
sample and the surface of the counter electrode within the vacuum
container, switching a gas supplied into the vacuum container to the
plasma doping gas after the plasma is generated, and supplying the
electric power to the sample electrode after the gas inside the vacuum
container has been switched to the plasma doping gas.

[0026] According to a fifth aspect of the present invention, there is
provided the plasma doping method as defined in the second aspect,
wherein, after the sample is placed on the sample electrode within the
vacuum container and before the electric power is supplied to the sample
electrode,

[0027] relatively moving the sample electrode and the counter electrode to
separate the sample electrode from the counter electrode such that a
distance G between the sample electrode and the counter electrode is
larger than a range defined by the equation (1), and in this state,
supplying the high-frequency electric power to the counter electrode
while a plasma doping gas is supplied into the vacuum container, gas is
exhausted from the vacuum container, and the inside of the vacuum
container is controlled to the plasma doping pressure, generating plasma
between the surface of the sample and the surface of the counter
electrode within the vacuum container, relatively moving the sample
electrode and the counter electrode after the plasma is generated to
restore a state where the distance G satisfies the equation (1), and
thereafter, supplying the electric power to the sample electrode.

[0028] According to a sixth aspect of the present invention, there is
provided the plasma doping method as defined in any one of the first to
fifth aspects, wherein a concentration of impurity material gas within
the gas introduced into the vacuum container is equal to or less than 1%.

[0029] According to a seventh aspect of the present invention, there is
provided the plasma doping method as defined in any one of the first to
fifth aspects, wherein a concentration of impurity material gas within
the gas introduced into the vacuum container is equal to or less than
0.1%.

[0030] According to an eighth aspect of the present invention, there is
provided the plasma doping method as defined in any one of the first to
seventh aspects, wherein the gas introduced to the vacuum container is a
mixed gas prepared by diluting an impurity material gas with a rare gas.
Further, as defined in a ninth aspect of the present invention, there is
provided the plasma doping method as defined in the eighth aspect,
wherein the rare gas is He.

[0031] With this structure, it is possible to realize the plasma doping
method with excellent reproducibility while realizing both accurate
control of the amount of dose and a low sputtering property.

[0032] According to tenth and eleventh aspects of the present invention,
there is provided the plasma doping method as defined in any one of the
first to ninth aspects, wherein the impurity material gas within the gas
is BxHy (x and y are natural numbers) or PxHy (x and y are natural
numbers).

[0033] With this structure, it is possible to prevent implantation of
undesirable impurities into the surfaces of samples.

[0034] According to a twelfth aspect of the present invention, there is
provided the plasma doping method as defined in any one of the first to
eleventh aspects, wherein the plasma doping processing is performed while
the gas is ejected toward the surface of the sample through gas ejection
holes provided in the counter electrode.

[0035] With this structure, it is possible to realize the plasma doping
method with more excellent reproducibility of the concentration of
impurities implanted to the sample surface.

[0036] Further, according to a thirteenth aspect of the present invention,
there is provided the plasma doping method as defined in any one of the
first to twelfth aspects, wherein the plasma doping processing is
performed in a state where the surface of the counter electrode is made
of silicon or a silicon oxide.

[0037] With this structure, it is possible to prevent implantation of
undesirable impurities into the surfaces of samples.

[0038] According to a fourteenth aspect of the present invention, there is
provided the plasma doping method as defined in any one of the first to
thirteenth aspects, wherein the plasma doping processing is performed in
a state where the sample is a semiconductor substrate made of silicon.
According to a fifteenth aspect of the present invention, there is
provided the plasma doping method as defined in any one of the first to
fourteenth aspects, wherein impurities in the impurity gas contained in
the gas is arsenic, phosphorus, or boron.

[0039] As the impurities, it is also possible to employ aluminum or
antimony.

[0040] According to a sixteenth aspect of the present invention, there is
provided a plasma doping apparatus comprising:

[0041] a vacuum container;

[0042] a sample electrode placed within the vacuum container;

[0043] a gas supply device for supplying gas into the vacuum container;

[0044] a counter electrode which is faced substantially in parallel to the
sample electrode;

[0045] an exhaust device for exhausting gas from the vacuum container;

[0046] a pressure control device for controlling a pressure within the
vacuum container; and

[0047] a power supply for supplying an electric power to the sample
electrode, wherein

[0048] a following equation (2) is satisfied, where S is an area of a
surface of the sample electrode, the surface being faced to the counter
electrode and also being a placement region of the surface in which the
sample is placed, and G is a distance between the sample electrode and
the counter electrode.

0.1 {square root over ((S/π))}G0.4 {square root over ((S/π))} (2)

[0049] With this structure, it is possible to realize the plasma doping
apparatus with excellent reproducibility of the concentration of
impurities implanted to the surfaces of samples.

[0050] According to a seventeenth aspect of the present invention, there
is provided the plasma doping apparatus as defined in the sixteenth
aspect, further comprising a high-frequency power supply for supplying a
high-frequency electric power to the counter electrode.

[0051] With this structure, it is possible to prevent the adsorption of
generated plasma to the counter electrode.

[0052] According to an eighteenth aspect of the present invention, there
is provided the plasma doping apparatus as defined in the seventeenth
aspect, wherein the pressure control device is capable of controlling the
pressure within the vacuum container in such a way as to switch between a
plasma doping pressure and a plasma generating pressure higher than the
plasma doping pressure,

[0053] after the sample is placed on the sample electrode within the
vacuum container and before the electric power is supplied to the sample
electrode, the high-frequency electric power is supplied from the
high-frequency power supply to the counter electrode while the pressure
within the vacuum container is maintained at the plasma generating
pressure which is higher than the plasma doping pressure by the pressure
control device, to generate plasma between the surface of the sample and
a surface of the counter electrode within the vacuum container, after the
plasma is generated, the pressure within the vacuum container is
gradually decreased to the plasma doping pressure by the pressure control
device, and after the pressure within the vacuum container reaches the
plasma doping pressure, the electric power is supplied from the power
supply to the sample electrode.

[0054] According to a nineteenth aspect of the present invention, there is
provided the plasma doping apparatus as defined in the seventeenth
aspect, wherein the gas supply device is capable of supplying the plasma
doping gas and plasma generating gas which causes discharge at a lower
pressure more easily than a dilution gas used for diluting an impurity
material gas in the plasma doping gas, in a switchable manner,

[0055] after the sample is placed on the sample electrode within the
vacuum container and before the electric power is supplied to the sample
electrode, the plasma generating gas which causes discharge at a lower
pressure more easily than the dilution gas used for diluting the impurity
material gas in the plasma doping gas is supplied into the vacuum
container by the gas supply device, and the high-frequency electric power
is supplied from the high-frequency power supply to the counter electrode
while the pressure within the vacuum container is maintained at a plasma
doping pressure by the pressure control device, to generate plasma
between the surface of the sample and the surface of the counter
electrode within the vacuum container, after the plasma is generated, the
gas supplied into the vacuum container is switched to the plasma doping
gas, and after the gas inside the vacuum container has been switched to
the plasma doping gas, the electric power is supplied to the sample
electrode.

[0056] According to a twentieth aspect of the present invention, there is
provided the plasma doping apparatus as defined in the seventeenth
aspect, further comprising a distance-adjustment driving device for
relatively moving the sample electrode with respect to the counter
electrode,

[0057] after the sample is placed on the sample electrode within the
vacuum container and before the electric power is supplied to the sample
electrode, the sample electrode and the counter electrode are moved
relative to each other, by the distance-adjustment driving device, to
separate the sample electrode from the counter electrode such that the
distance G between the sample electrode and the counter electrode is
larger than a range defined by the equation (2), and in this state, the
high-frequency electric power is supplied from the high-frequency power
supply to the counter electrode while a plasma doping gas is supplied
into the vacuum container, gas is exhausted from the vacuum container,
and the inside of the vacuum container is controlled to a plasma doping
pressure to generate plasma between the surface of the sample and the
surface of the counter electrode within the vacuum container, after the
plasma is generated, the sample electrode and the counter electrode are
moved relative to each other by the distance-adjustment driving device to
restore a state where the distance G satisfies the equation (2), and
thereafter, the electric power is supplied to the sample electrode.

[0058] According to a twenty-first aspect of the present invention, there
is provided the plasma doping apparatus as defined in any one of the
sixteenth to twentieth aspects, wherein the gas supply device is adapted
to supply the gas through gas ejection holes provided in the counter
electrode.

[0059] With this structure, it is possible to realize the plasma doping
apparatus with more excellent reproducibility of the concentration of
impurities implanted to the surfaces of samples.

[0060] Further, according to a twenty-second aspect of the present
invention, there is provided the plasma doping apparatus as defined in
any one of the sixteenth to twenty-first aspects, wherein the surface of
the counter electrode is made of silicon or a silicon oxide.

[0061] With this structure, it is possible to prevent implantation of
undesirable impurities into the surfaces of samples.

[0062] According to a twenty-third aspect of the present invention, there
is provided a plasma doping method comprising:

[0063] placing a sample on a sample electrode within a vacuum container;

[0064] relatively moving the sample electrode and the counter electrode to
separate the sample electrode from the counter electrode such that a
distance G between the sample electrode and the counter electrode
opposite the sample electrode is larger than a distance for plasma doping
processing, and in this state, supplying the high-frequency electric
power to the counter electrode while supplying a plasma doping gas into
the vacuum container, exhausting a gas from the vacuum container, and
controlling an inside of the vacuum container to a plasma doping
pressure, to generate plasma between a surface of the sample and a
surface of the counter electrode within the vacuum container;

[0065] after the plasma is generated, relatively moving the sample
electrode and the counter electrode to restore the distance G to a
distance for plasma doping processing, and thereafter, supplying the
electric power to the sample electrode; and

[0066] performing plasma doping processing to implant impurities into the
surface of the sample, in a state where the distance G between the sample
electrode and the counter electrode is maintained at the distance for
plasma doping processing, where S is an area of the surface which is
faced to the counter electrode, out of surfaces of the sample.

BRIEF DESCRIPTION OF DRAWINGS

[0067] These and other aspects and features of the present invention will
become clear from the following description taken in conjunction with the
preferred embodiments thereof with reference to the accompanying
drawings, in which:

[0068] FIG. 1A is a cross-sectional view illustrating the structure of a
plasma doping apparatus for use in a first embodiment of the present
invention;

[0069]FIG. 1B is an enlarged cross-sectional view illustrating the
structure of a sample electrode in the plasma doping apparatus for use in
the first embodiment of the present invention;

[0070]FIG. 2 is a graph illustrating comparison between the relationship
between the number of processed substrates and the surface resistance
according to the first embodiment of the present invention and such a
relationship in the prior art;

[0071]FIG. 3 is a cross-sectional view illustrating the structure of a
plasma doping apparatus for use in a modification of the first embodiment
of the present invention;

[0072]FIG. 4 is a cross-sectional view illustrating the structure of a
plasma doping apparatus for use in another modification of the first
embodiment of the present invention; and

[0073]FIG. 5 is a cross-sectional view illustrating the structure of a
plasma doping apparatus used in a conventional example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0074] Before the description of the present invention proceeds, it is to
be noted that like parts are designated by like reference numerals
throughout the accompanying drawings.

[0075] Hereinafter, embodiments of the present invention will be described
in detail, with reference to the drawings.

First Embodiment

[0076] Hereinafter, a first embodiment of the present invention will be
described with reference to FIGS. 1A to 2.

[0077] A plasma doping apparatus according to the first embodiment of the
present invention is a plasma doping apparatus including a vacuum
container (vacuum chamber) 1, a sample electrode (first electrode) 6
placed within the vacuum container 1, a gas supply device 2 for supplying
plasma doping gas into the vacuum container 1, a counter electrode
(second electrode) 3 which is placed within the vacuum container 1 and is
opposed substantially in parallel to the sample electrode 6, a turbo pump
8 serving as one example of an exhaust device for exhausting gas in the
vacuum container 1, a pressure adjustment valve 9 serving as one example
of a pressure control device for controlling the pressure within the
vacuum container 1, and a sample-electrode high-frequency power supply 12
serving as one example of a power supply for supplying a high-frequency
power to the sample electrode, as illustrated in the cross-sectional
views of FIGS. 1A and 1B, wherein it is characterized in that the
distance G between the sample electrode 6 and the counter electrode 3 is
set to be sufficiently smaller than the area S of the surface of the
sample electrode 6 which is opposed to the counter electrode 3 with the
areas being the placement region in which a substrate (more specifically,
a silicon substrate) 7 as one example of a sample is to be placed, so as
to prevent plasma generated between the sample electrode 6 and the
counter electrode 3 from being diffused to the outside of the space
between the sample electrode 6 and the counter electrode 3 and also so as
to confine the plasma substantially within the space between the sample
electrode 6 and the counter electrode 3. Further, in this case, the area
of the sample electrode 6 means the area of the substrate placement
surface (the area of the exposed portion which is not covered with an
insulation member 6B in FIG. 1B) and does not include the areas of the
side surface portions of the sample electrode 6. In FIG. 1A, the sample
electrode 6 is schematically illustrated as having a rectangular
cross-section. One example of the sample electrode 6 has an upper portion
having a smaller diameter and having a substrate placement surface at its
upper end surface and a lower portion having a protruding portion with a
diameter greater than the diameter of the upper portion, and thus is
structured to have an upward convex shape, as illustrated in the
cross-sectional view of FIG. 1B. In FIG. 1B, 6B designates an insulation
member which is made of an insulation material and covers the portion of
the upper portion of the sample electrode 6 other than the substrate
placement surface. 6C designates an aluminum ring which is grounded and
is coupled to supporting columns 10 which will be described later. In
FIG. 1B, as an example, the substrate 7 is illustrated as being greater
than the substrate placement surface which is the upper end surface of
the sample electrode 6 but being smaller than the protruding portion of
the lower portion of the sample electrode 6.

[0078] That is, referring to FIG. 1A, in the plasma doping apparatus, a
predetermined gas (plasma doping gas) is introduced into a gas reservoir
4 provided within the counter electrode 3 within the vacuum container 1
from the gas supply device 2, and then the gas is ejected toward the
substrate 7 as an example of the sample placed on the sample electrode 6,
through a number of gas ejection holes 5 provided in the counter
electrode 3. The counter electrode 3 is placed such that its surface (the
lower surface in FIG. 1A) is faced to the surface of the sample electrode
6 (the upper surface in FIG. 1A) substantially in parallel thereto.

[0079] Further, the gas supplied from the gas supply device 2 to the
vacuum container 1 is exhausted from the vacuum container 1 by the turbo
molecular pump 8 as an example of the exhaust device through an exhaust
opening 1a, and also the degree of opening of the exhaust opening 1a is
adjusted by the pressure adjustment valve 9 as an example of the pressure
control device, so that the pressure within the vacuum container 1 is
maintained at a predetermined pressure (a plasma doping pressure).
Further, the turbo molecular pump 8 and the exhaust opening 1a are placed
just below the sample electrode 6, and also the pressure adjustment valve
9 is a liftable valve positioned just below the sample electrode 6 and
just above the turbo molecular pump 8. Furthermore, the sample electrode
6 is fixed at a middle portion of the vacuum container 1 with the four
insulation supporting columns 10. By supplying a high-frequency electric
power with a frequency of 60 MHz to the counter electrode 3 from the
counter-electrode high-frequency power supply 11, it is possible to
generate capacitive-coupled plasma between the counter electrode 3 and
the sample electrode 6. Further, there is provided the sample-electrode
high-frequency power supply 12 for supplying a high-frequency electric
power with a frequency of 1.6 MHz to the sample electrode 6, and the
sample-electrode high-frequency power supply 12 functions as a
bias-voltage source which controls the electric potential of the sample
electrode 6 such that the substrate 7 as an example of the sample is
maintained at a negative potential with respect to the plasma. Instead of
using the sample-electrode high-frequency power supply 12, a pulse power
supply can also be used to supply a pulse power to the sample electrode 6
to control the potential of the substrate 7. An insulation member 13 is
for galvanically isolating the counter electrode 3 from the vacuum
container 1 which is grounded. In this manner, by accelerating ions
within plasma toward the surface of the substrate 7 as an example of the
sample to cause these ions to impinge thereon, it is possible to treat
the surface of the substrate 7 as an example of the sample. By using a
gas containing diborane or phosphine as the plasma doping gas, it is
possible to perform the plasma doping processing.

[0080] In a case of performing the plasma doping processing, the flow
rates of gases each including an impurity material gas are controlled to
predetermined values, by flow-rate control devices (mass-flow
controllers) (for example, first to third mass-flow controllers 31, 32,
and 33 in FIG. 3 which will be described later) which are provided within
the gas supply device 2 in FIG. 1A. Generally, a gas prepared by diluting
an impurity material gas with helium, such as a gas prepared by diluting
diborane (B2H6) to 0.5% with helium (He), is used as the
impurity material gas, and the flow rate of this gas is controlled by the
first mass-flow controller (for example, the first mass-flow controller
31 in FIG. 3 which will be described later). Further, the flow rate of
helium is controlled by the second mass-flow controller (e.g., the second
mass-flow controller 32 in FIG. 3 which will be described later).
Further, these gases controlled in flow rate by the first and second
mass-flow controllers are mixed with each other in the gas supply device
2, and thereafter, the mixed gas is introduced into the gas reservoir 4
through a pipe 2p. The impurity material gas which has been adjusted to
have a predetermined concentration is supplied from the gas reservoir 4
to the gap between the counter electrode 3 and the sample electrode 6
within the vacuum container 1, through the number of gas ejection holes
5.

[0081] Further, in FIG. 1A, 80 designates a control device for controlling
plasma doping processing, and this control device 80 controls the
respective operations of the gas supply device 2, the turbo molecular
pump 8, the pressure adjustment valve 9, the counter-electrode
high-frequency power supply 11, and the sample-electrode high-frequency
power supply 12 for performing the predetermined plasma doping
processing.

[0082] As an actual example, the substrate 7 used herein is a silicon
substrate with a circular shape (having a notch at a portion thereof) and
a diameter of 300 mm. Further, there will be described in the following,
as an example, plasma doping processing in the case where the distance G
between the sample electrode 6 and the counter electrode 3 is set to 25
mm.

[0083] In performing plasma doping using the aforementioned plasma
processing apparatus, at first, the inner walls of the vacuum container 1
including the surface of the counter electrode 3 are cleaned using water
and an organic solvent.

[0084] Next, a substrate 7 is placed on the sample electrode 6.

[0085] Next, a high-frequency electric power of 1600 W is supplied from
the counter-electrode high-frequency power supply 11 to the counter
electrode 3, while the temperature of the sample electrode 6 is
maintained at, for example, 25 C.°. B2H6 gas diluted
with He, and He gas, for example, are supplied at flow rates of 5 sccm
and 100 sccm, respectively, from the gas supply device 2 into the vacuum
container 1, and also, the pressure within the vacuum container 1 is
maintained at 0.8 Pa by the pressure adjustment valve 9, to generate
plasma between the counter electrode 3 and the substrate 7 on the sample
electrode 6 within the vacuum container 1. Also, a high-frequency
electric power of 140 W is supplied from the sample-electrode
high-frequency power supply 12 to the sample electrode 6 for 50 seconds
to cause boron ions within the plasma to impinge on the surface of the
substrate 7, thus implanting boron to the vicinity of the surface of the
substrate 7. Then, the substrate 7 is taken out from the vacuum container
1 and activated, and thereafter, the surface resistance (a value relating
to the amount of dose) is measured.

[0086] Under the same conditions, plasma doping processing is successively
applied to the substrates 7. As a result, first several substrates
exhibit decreasing surface resistance after activation, and the
substrates subsequent thereto exhibit a substantially constant surface
resistance, as illustrated by a curve "a" in FIG. 2.

[0087] Further, after the surface resistance reaches a substantially
constant value, the surface resistance is varied within an extremely
small width.

[0088] For comparison, the same processing is conducted using an
inductively-coupled plasma source as in the prior-art example (in the
prior-art example, the distance between the quartz plate which is
dielectric and the electrode is in the range of 200 mm to 300 mm). As a
result, first several tens of substrates exhibit moderately-decreasing
surface resistance, and the substrates subsequent thereto exhibit surface
resistance asymptotically approaching a constant value, as illustrated by
a curve "b" in FIG. 2.

[0089] Further, in the prior-art example, after the surface resistance
substantially reach a constant value, the surface resistance is varied
within a relatively large variation width, which is several times the
variation width of the present first embodiment.

[0090] Hereinafter, there will be described reasons for the fact that the
aforementioned difference is observed.

[0091] In the prior-art example, during successively performing the plasma
doping processing just after the cleaning of the inner wall of the vacuum
container 1, a thin film containing boron is gradually deposited on the
inner wall surface of the vacuum container 1. It is considered that this
phenomenon occurs since boron-based radicals (neutral particles) produced
within the plasma are adsorbed to the inner wall surface of the vacuum
container, and also boron-based ions are accelerated by the potential
difference between the plasma potential (=approximately 10 to 40 V) and
the potential of the inner wall of the vacuum container (usually, since
the inner wall of the vacuum container is dielectric, a floating
potential=approximately 5 to 20 V) and then impinge on the inner wall
surface of the vacuum container, so that a thin film containing boron is
grown thereon due to thermal energy or ion impingement energy. It is
considered that, along with the increase in the thickness of this
deposited film, the probability of adsorption of boron-based radicals to
the inner wall surface of the vacuum container is gradually decreased,
and therefore, the density of boron-based radicals within the plasma is
gradually increased, in the case of using B2H6 as a doping
material gas. Further, ions within the plasma are accelerated by the
aforementioned potential difference and then impinge on the boron-based
thin film deposited on the inner wall surface of the vacuum container,
which causes sputtering, thereby gradually increasing the amount of
particles containing boron which are supplied to the plasma.
Consequently, the amount of dose is gradually increased, which gradually
decreases the surface resistance after activation. Further, the
temperature of the inner wall surface of the vacuum container is varied
along with the generation of plasma or the stoppage thereof, which varies
the probability of adsorption of boron-based radicals to the inner wall
surface, thereby causing the surface resistance after activation to be
largely varied.

[0092] On the other hand, in the present first embodiment, the distance G
between the sample electrode 6 and the counter electrode 3 is as small as
25 mm as compared with the area of the sample electrode 6 in which a
wafer with a diameter of 300 mm as an example of the substrate 7 is
placed, so that so-called narrow-gap discharge is caused. Further, the
processing is performed while the gas is ejected toward the surface of
the substrate 7 through the gas ejection holes 5 provided in the counter
electrode 3. In this case, the surface condition of the inner wall
surface of the vacuum container 1 (except the surface of the counter
electrode 3) exerts significantly small influence on the density of
boron-based radicals and the density of boron ions within the plasma.
This is mainly for the following four reasons.

[0093] (1) Due to the narrow-gap discharge, the plasma is mainly generated
only between the counter electrode 3 and the substrate 7, and therefore,
boron-based radicals are very unlikely to be adsorbed to the inner wall
surface of the vacuum container 1 (except the surface of the counter
electrode 3), so that a thin film containing boron is less likely to be
deposited thereon.

[0094] (2) The area of the inner wall surface of the vacuum container 1
(except the surface of the counter electrode 3) relative to the substrate
7 is smaller than that of the prior-art example, which reduces the
influence of the inner wall surface of the vacuum container 1.

[0095] (3) Due to the application of the high-frequency electric power to
the counter electrode 3, a self-bias voltage is generated at the surface
of the counter electrode 3, and therefore, boron-based radicals are very
unlikely to be adsorbed thereto, so that the condition of the surface of
the counter electrode 3 is hardly changed even when the doping processing
is successively performed.

[0096] (4) The gas is flowed along the surface of the substrate 7 in a
single direction from the center of the substrate 7 to the periphery
thereof, which attenuates the influence of the inner wall surface of the
vacuum container 1 on the substrate 7.

[0097] Further, the present inventors determine a preferable range for the
distance between the sample electrode 6 and the counter electrode 3.
Assuming that the area of the surface of the substrate 7 (the surface
which is faced to the counter electrode 3 or the surface of the sample
electrode 6 which is faced to the counter electrode 3 and also the
placement region on which the substrate 7 is to be placed) is S, in the
case where the substrate 7 has a circular shape, the radius thereof is
(S/π)-1/2. Assuming that the distance between the sample
electrode 6 and the counter electrode 3 is G, under a condition where the
following equation (3) holds, namely under a condition where the
inter-electrode distance G falls within the range of 0.1 time to 0.4 time
the radius of the substrate 7, a preferable impurity concentration
reproducibility is obtained.

0.1 {square root over ((S/π))}G0.4 {square root over ((S/π))} (3)

When the inter-electrode distance G is excessively small (smaller than
0.1 time the radius), plasma could not be generated within a pressure
range suitable for performing the plasma doping (equal to or less than 3
Pa). On the contrary, when the inter-electrode distance G is excessively
large (larger than 0.4 time the radius), several tens of substrates were
required until the surface resistance after activation is stabilized just
after wet cleaning, as in the prior-art example. Further, after the
surface resistance is substantially stabilized, the surface resistance is
varied within a large variation width.

[0098] As described above, generating the narrow-gap discharge through the
application of the high-frequency electric power to the counter electrode
3 using the high-frequency power supply 11 is extremely important in
ensuring the processing reproducibility. This is a particularly prominent
phenomenon in plasma doping. In a case where the variation in etching
property due to the deposition of a carbon-fluoride-based thin film on
the inner wall of the vacuum container is problematic in applying dry
etching to an insulation film, narrow-gap discharge may be utilized,
wherein the concentration of carbon-fluoride-based gas within mixed gas
introduced into the vacuum container is about several percentages, and
the influence of the deposited film is relatively small. On the other
hand, in the case of the plasma doping, the concentration of impurity
material gas within inert gas introduced into the vacuum container is 1%
or less (0.1% or less, particularly in a case where it is desired to
control the amount of dose with higher accuracy), which causes the
influence of the deposited film to be relatively large. In the case where
the concentration of impurity material gas within inert gas exceeds 1%,
it is impossible to provide a so-called self-regulation effect, thereby
inducing malfunction that the amount of dose cannot be controlled
accurately. Accordingly, the concentration of impurity material gas
within inert gas is set to be 1% or less. It is necessary that the
concentration of impurity material gas within inert gas introduced into
the vacuum container be equal to or more than 0.001%. If it is smaller
than 0.001%, processing should be performed for an extremely long time to
attain a desired amount of dose.

[0099] Further, the use of the present invention offers the advantage of
improvement in the accuracy of controlling the amount of dose, dose
monitoring utilizing in-situ monitoring techniques such as emission
spectroscopy and mass spectrometry, and the like. This is because of the
following reason. That is, it is known that the saturation amount of dose
in the so-called self-regulation phenomenon depends on the concentration
of impurity material gas within mixed gas introduced into the vacuum
container, wherein the self-regulation phenomenon is a phenomenon that,
in processing a single substrate, the amount of dose is saturated along
with the elapse of processing time. According to the present invention,
it is possible to obtain relatively easily measurement values strongly
relating to particles such as ions and radicals generated by dissociation
or electrolytic dissociation of impurity material gas within plasma
through in-situ monitoring, regardless of the condition of the inner wall
of the vacuum container.

[0100] Further, in the plasma doping apparatus described in the Patent
Document 4, the counter electrode (anode) provided opposite to the sample
is maintained at a ground electric potential, which causes a thin film
containing boron to be deposited on the counter electrode, when plasma
doping processing is successively performed. Further, the Patent Document
4 only describes that the distance (gap) between the counter electrode
(anode) and the sample electrode (cathode) "can be adjusted with respect
to different voltages".

[0101] In the aforementioned first embodiment of the present invention,
there have been exemplified only portions of various variations of the
shape of the vacuum container 1, the structure and placement of the
electrodes 3 and 6, and the like, within the applicable scope of the
present invention. It goes without saying that the present invention can
be implemented in various variations, as well as the aforementioned
examples.

[0102] Further, there has been exemplified a case where the high-frequency
electric power with a frequency of 60 MHz is supplied to the counter
electrode 3, and where the high-frequency electric power with a frequency
of 1.6 MHz is supplied to the sample electrode 6, these frequencies are
merely illustrative. A preferable frequency of the high-frequency
electric power supplied to the counter electrode 3 is substantially
within the range of 10 MHz to 100 MHz. If the frequency of the
high-frequency electric power supplied to the counter electrode 3 is
lower than 10 MHz, it is impossible to provide a sufficient plasma
density. On the contrary, if the frequency of the high-frequency electric
power supplied to the counter electrode 3 is higher than 100 MHz, it is
impossible to provide a sufficient self-bias voltage, which tends to
cause a thin film containing impurities to be deposited on the surface of
the counter electrode 3.

[0103] A preferable frequency of the high-frequency electric power
supplied to the sample electrode 6 is substantially within the range of
300 kHz to 20 MHz. If the frequency of the high-frequency electric power
supplied to the sample electrode 6 is lower than 300 kHz, it is
impossible to attain high-frequency matching easily. On the contrary, if
the frequency of the high-frequency electric power supplied to the sample
electrode 6 is higher than 20 MHz, this will tend to induce an in-plain
distribution in the voltage applied to the sample electrode 6, thereby
degrading the uniformity of doping processing.

[0104] Further, the surface of the counter electrode 3 can be made of
silicon or a silicon oxide, which can prevent the implantation of
undesirable impurities into the surface of a silicon substrate as an
example of the substrate 7.

[0105] Further, in the case where the substrate 7 is a semiconductor
substrate made of silicon, the substrate 7 can be utilized in fabrication
of fine transistors, by using arsenic, phosphorus, or boron as the
impurities. Also, the substrate 7 may be made of a compound
semiconductor. Aluminum or antimony can be used as the impurities.

[0106] Further, a known heater and a known cooling device can be
incorporated to respectively control the temperature of the inner wall of
the vacuum container 1 and the temperatures of the counter electrode 3
and the sample electrode 6, which enables controlling, with higher
accuracy, the probability of adsorption of impurity radicals to the inner
wall of the vacuum container 1, the counter electrode 3, and the surface
of the substrate 7, thereby further increasing the reproducibility.

[0107] Further, while there has been exemplified a case where a mixed gas
prepared by diluting B2H6 with He is used as plasma doping gas
to be introduced into the vacuum container 1, generally, it is also
possible to use a mixed gas prepared by diluting an impurity material gas
with a rare gas. As an impurity material gas, it is possible to use BxHy
(x and y are natural numbers) or PxHy (x and y are natural numbers).
These gases have the advantage of containing, as impurities, only H which
will have less influence on the substrate even if it is intruded into the
substrate, in addition to B or P. It is also possible to use other gasses
containing B, such as BF3, BCl3, or BBr3. Also, it is
possible to use other gasses containing P, such as PF3, PF5,
PCl3, PCl5, or POCl3. Further, He, Ne, Ar, Kr, Xe, or the
like can be used as the rare gas, but He is most preferable. This is for
the following reason. The use of He can prevent the implantation of
undesirable impurities into the surfaces of samples and also can realize
a plasma doping method with excellent reproducibility while realizing
both accurate control of the amount of dose and a low sputtering
property. By using a mixed gas prepared by diluting an impurity material
gas with a rare gas, it is possible to significantly reduce the change in
the amount of dose caused by the film containing impurities such as boron
which has been formed on the chamber inner wall. This enables controlling
the distribution of the amount of dose with higher accuracy, by
controlling the gas ejection distribution. This makes it easier to ensure
preferable in-plain uniformity of the amount of dose. Ne is the most
preferable rare gas next to He. Ne has the advantage of easily causing
discharge at a low pressure, while having the drawback of having a
sputtering rate slightly higher than He.

[0108] It should be noted that the present invention is not limited to the
first embodiment and can be implemented in various modes.

[0109] For example, while, in the first embodiment, there has been
exemplified a case where B2H6 gas diluted with He, and He gas
are supplied from the gas supply device 2 at flow rates of 5 sccm and 100
sccm, respectively, and the high-frequency electric power of 1600 W is
supplied to the counter electrode 3 from the counter-electrode
high-frequency power supply 11 while the pressure within the vacuum
container 1 is maintained at 0.8 Pa by the pressure adjustment value 9,
thus generating plasma between the counter electrode 3 and the substrate
7 on the sample electrode 6 within the vacuum container 1, there are
cases where it is difficult to generate plasma at a low pressure in a
state where the partial pressure of He gas is high. In this case, it is
effective to appropriately employ the following methods as modifications
of the first embodiment of the present invention.

[0110] A first method is a method which changes the pressure. At first, a
high-frequency electric power is supplied to the counter electrode 3 from
the counter-electrode high-frequency power supply 11, while the pressure
within the vacuum container 1 is maintained, through the pressure
adjustment valve 9, at a plasma-generating pressure which is equal to or
higher than 1 Pa (typically, 10 Pa) and higher than the plasma doping
pressure, to generate plasma between the counter electrode 3 and the
substrate 7 on the sample electrode 6 within the vacuum container 1. At
this time, the sample electrode 6 is not supplied with a high-frequency
electric power from the sample-electrode high-frequency power supply 12.
After the plasma is generated, the pressure within the vacuum container 1
is gradually reduced to the plasma doping pressure which is equal to or
lower than 1 Pa (typically, 0.8 Pa), by adjusting the pressure adjustment
valve 9. A similar procedure can be possibly used in the case of using a
so-called high-density plasma source such as an ECR (electron cyclotron
resonance plasma source) or an ICP (inductively coupled plasma source).
However, in the structure of the apparatus according to the modification
of the first embodiment of the present invention, the volume of plasma is
significantly smaller than that in the case of using a high-density
plasma source, and accordingly, it is necessary to decrease the pressure
more slowly by the pressure adjustment valve 9 in order to prevent the
generated plasma from being lost. However, if the pressure is decreased
excessively slowly, this will extend the total processing time and also
may cause contamination on the substrate 7. Accordingly, it is preferable
to decrease the pressure by taking about 3 to 15 seconds using the
pressure adjustment valve 9. After the pressure within the vacuum
container 1 is decreased to the plasma doping pressure, a high-frequency
electric power is supplied to the sample electrode 6 from the
sample-electrode high-frequency power supply 12.

[0111] A second method is a method which changes the types of gases. As
illustrated in FIG. 3, the gas supply device 2 is constituted by, for
example, the first to third mass-flow controllers 31, 32, and 33 which
are controlled and operated by the control device 80, first to third
valves 34, 35, and 36 which are controlled and operated by the control
device 80, and first to third bottles 37, 38, and 39. The first bottle 37
stores B2H6 gas diluted with He, the second bottle 38 stores He
gas, and the third bottle 39 stores Ne gas. Then, at first, Ne gas, which
is an example of a plasma-generating gas which can cause discharge at a
lower pressure more easily than He, is supplied from the third bottle 39
into the vacuum container 1, through the third valve 38, the third
mass-flow controller 33, and the pipe 2p, by opening the third valve 38
while closing the first and second valves 34 and 35. The flow rate of Ne
gas from the third bottle 39 is maintained at a constant value by the
third mass-flow controller 33. At this time, the flow rate of Ne gas is
set to be substantially the same as the gas flow rate at the later step
of supplying the high-frequency electric power to the sample electrode 6.
The high-frequency electric power is supplied from the counter-electrode
high-frequency power supply 11 to the counter electrode 3 while the
pressure within the vacuum container 1 is maintained at 0.8 Pa by the
pressure adjustment valve 9, to generate plasma between the counter
electrode 3 and the substrate 7 on the sample electrode 6 within the
vacuum container 1. At this time, the sample electrode 6 is not supplied
with the high-frequency electric power. After the plasma is generated,
the gas supplied into the vacuum container 1 through the first and second
valves 34 and 35, the first and second mass-flow controllers 31 and 32,
and the pipe 2p from the first and second bottles 37 and 38 is changed to
the mixed gas constituted by He and B2H6 gas, by opening the
first and second valves 34 and 35 while closing the third valve 38. The
flow rates of these gases are maintained at constant values by the first
and second mass-flow controllers 31 and 32. After the types of gases are
changed, the high-frequency electric power is supplied to the sample
electrode 6 from the sample-electrode high-frequency power supply 12. A
similar procedure can be possibly used in the case of using a so-called
high-density plasma source such as an ECR (electron cyclotron resonance
plasma source) or an ICP (inductively-coupled plasma source). However, in
the structure of the apparatus according to the present invention, the
volume of plasma is significantly smaller than that in the case of using
the high-density plasma source, and accordingly, it is preferable to
change the type of gas more slowly in order to prevent the generated
plasma from being lost. However, if the type of gas is changed
excessively slowly, it will extend the total processing time and also may
cause contamination on the substrate 7. Accordingly, it is preferable to
change the type of gas by taking about 3 to 15 seconds. In order to
change the type of gas slowly, the set flow-rate values of the first and
second mass-flow controllers 31 and 32 are set to zero or an
extremely-small value (10 sccm or less) at the moment of opening the
first and second valves 34 and 35, and then these set flow-rate values
are controlled such that the flow rates are gradually increased. Further,
after the first and second valves 34 and 35 are opened, the set flow-rate
value of the third mass-flow controller 33 is gradually reduced while the
third valve 33 is kept open, and after the set flow-rate value of the
third mass-flow controller 33 reaches zero or an extremely-small value
(10 sccm or less), the third valve 36 is closed.

[0112] A third method is a method which changes the distance G between the
sample electrode 6 and the counter electrode 3. As another modification
of the first embodiment, in order to move the sample electrode 6 and the
counter electrode 3 relative to each other to control the distance G
between the sample electrode 6 and the counter electrode 3, for example,
as illustrated in FIG. 4, there is provided a bellows 40 as an example of
a distance-adjustment driving device (such as a sample-electrode
lifting/lowering driving device) between the bottom surface of the vacuum
container 1 and the sample electrode 6 within the vacuum container 1 (or
as an example of a distance-adjustment driving device (such as a
counter-electrode lifting/lowering driving device) between the upper
surface of the vacuum container 1 and the counter electrode 3 within the
vacuum container 1, in the case of lifting or lowering the counter
electrode). Further, there is provided a fluid supply device 40a for
supplying, to the bellows 40, a fluid for expanding or contracting the
bellows 40, such that the sample electrode 6 (or the counter electrode 3)
can be lifted or lowered freely within the vacuum container 1 through the
bellows 40 by driving the fluid supply device 40a through the operation
control by the control device 80. In this case, the pressure adjustment
valve 9 and the pump 8 are provided on a side surface of the vacuum
container 1 (not illustrated). In the apparatus having such a structure,
at first, the sample electrode 6 is lowered (or the counter electrode 3
is lifted), by driving the fluid supply device 40a, to set the distance G
to the plasma generating distance of, for example, 80 mm, which is
greater than the plasma-doping distance. In this state, B2H6
gas diluted with He, and He gas are supplied from the gas supply device 2
to the vacuum container 1, and the high-frequency electric power is
supplied to the counter electrode 3 from the counter-electrode
high-frequency power supply 11 while the pressure within the vacuum
container 1 is maintained at 0.8 Pa by the pressure adjustment value 9,
to generate plasma between the counter electrode 3 and the substrate 7 on
the sample electrode 6 within the vacuum container 1. At this time, the
sample electrode 6 is not supplied with the high-frequency electric
power. After the plasma is generated, the sample electrode 6 is lifted
(or the counter electrode 3 is lowered), by driving the fluid supply
device 40a, to change the distance G to 25 mm. The generation of the
plasma may be automatically detected by detecting plasma light emission
with a detector, through a window provided in the vacuum container 1. In
this case, the fluid supply device 40a may be driven on the basis of
detection signals from the detector. More simply, a time period
sufficient to generate the plasma may be preliminarily set, and after the
elapse of the plasma generating preset time period, the fluid supply
device 40a may be driven on the assumption that the plasma has been
generated. After the distance G is set to be 25 mm, the driving of the
fluid supply device 40a is stopped, and the high-frequency electric power
is supplied to the sample electrode 6 from the sample-electrode
high-frequency power supply 12. If the distance G is changed excessively
abruptly, the generated plasma may be lost. On the contrary, if the
distance G is changed excessively slowly, this will extend the total
processing time and also may cause contamination on the substrate 7.
Accordingly, it is preferable to change the distance G by taking about 3
to 15 seconds. While, in the present modification, there has been
exemplified a case where the distance G is set to 80 mm in the step of
generating the plasma at first, it is preferable to generate the plasma
in a state where the following equation (4) is satisfied.

0.4 S π < G < S π ( 4 ) ##EQU00001##

If the distance G is excessively small (smaller than 0.4 time the
radius), plasma may not be generated. On the contrary, if the distance G
is excessively large (larger than 1.0 time the radius), this will
excessively increase the volume of the vacuum container 1, resulting in
insufficient pump exhaust ability.

[0113] Also, two or more methods out of the aforementioned three methods
may be combined.

[0114] Note that, in the case of using an ICP (inductively-coupled plasma
source), in order to reduce the number of substrates required until the
surface resistance after activation is stabilized from just after the wet
cleaning is finished, it is effective to perform processing in a state
where the distance G between the sample electrode 6 and the dielectric
window facing to the sample electrode 6 satisfies the following equation
(5).

0.1 S π < G < 0.4 S π ( 5 )
##EQU00002##

[0115] Also, in the aforementioned modification, the bellows 40 as an
example of the sample-electrode lifting/lowering driving device may be
provided between the bottom surface of the vacuum container 1 and the
sample electrode 6 within the vacuum container 1, and also, the bellows
40 as an example of the counter-electrode lifting/lowering driving device
may be provided between the upper surface of the vacuum container 1 and
the counter electrode 3 within the vacuum container 1 for lifting and
lowering the counter electrode. Thus, both the sample electrode 6 and the
counter electrode 3 may be moved to move the sample electrode 6 and the
counter electrode 3 relative to each other, in order to control the
distance G between the sample electrode 6 and the counter electrode 3.

[0116] Also, in the case where the present invention is applied to an ECR
(electron cyclotron resonance plasma source) or an ICP
(inductively-coupled plasma source), the distance between the counter
electrode and a dielectric plate or a surface including gas ejection
holes may be set as G, instead of setting the distance between the sample
electrode and the aforementioned counter electrode as G.

[0117] Further, while, in the present invention, the distance G has been
described as being the distance between the electrodes, it is necessary
that the distance G be defined as the distance between the substrate and
the electrode in a strict sense. However, the substrate is significantly
smaller than the distance, and accordingly, there is no problem in
describing the distance G as the distance between the electrodes without
taking into consideration the thickness of the substrate in the
embodiments and examples.

[0118] By properly combining the arbitrary embodiments of the
aforementioned various embodiments, the effects possessed by the
embodiments can be produced.

INDUSTRIAL APPLICABILITY

[0119] According to the present invention, there are provided a plasma
doping method and apparatus having excellent reproducibility of the
concentration of impurity implanted into the surfaces of samples.
Accordingly, the present invention can be applied to fabrication of
thin-film transistors for use in liquid crystals and the like, including
impurity doping processing for semiconductor devices.

[0120] Although the present invention has been fully described in
connection with the preferred embodiments thereof with reference to the
accompanying drawings, it is to be noted that various changes and
modifications are apparent to those skilled in the art. Such changes and
modifications are to be understood as included within the scope of the
present invention as defined by the appended claims unless they depart
therefrom.